Detection system



Patented Nov. 15, 1949 DETECTION SYSTEM Thaddeus Sloncxewski, GlenwoodLanding, N. Y.,

assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application September 25, 1945, Serial No.618,551

'l Claims. (Cl. FZ5- 183) This invention relates to magnetic measuringand more particularly to a lsystem of great sensitivity for measuringminute changes in the strength of a magnetic field.

It is a well-known fact that the earths magnetic field over any givenlimited area is substantially uniform except that this uniformity maysuffer distortion in the presence of paramagnetic or diamagneticmaterial. This distortion usually results in a change in both thedirection and absolute intensity of the eld. In most cases theparamagnetlc or diamagnetic body which produces the magnetic distortionis located at a considerable distance from the field strength measuringor .indicating device and if thedistorting material is to be detectedthe measuring or indicating device must be highly sensitive and wellcompensated against extraneous iniiuences.

In a copending patent application of T. Sionczewski filed April 20,1943, Serial No. 483,756, now Patent No. 2,485,931, issued October 25,1949, there is disclosed a detector system for indicating the intensityof a magnetic field irrespective of the position taken by the detectingelement in space. That system employed three magnetic elements mountedwith their principal magnetic axes mutually perpendicular. They wereexcited with an alternating magnetomotive force of constant intensity togenerate even order harmonics in windings on each magnetometer whichwere found to be proportional to the product of the absolute iieldstrength and the cosine of the angle between the principal axis of eachmagnetometer and the direction of the eld. An even order harmonic wasselected from each magnetometer, squared in a squaring circuit and thethree squared output currents or voltages were added together andmeasured by a -direct current measuring means. It was shown that thereading of the direct current measuring means was a measure of theabsolute total iield intensity irrespective of the orientation of thedetector elements in space.

In another copending application of E. P. Felch and T. Slonczewski alsofiled April 20, 1943, Serial No. 483,755, now Patent No. 2,468,968, itis shown that for practical reasons if one of the three mutuallyperpendicular elements is kept in substantial alignment with the iield,considerable improvement in accuracy can be achieved even though it istheoretically unnecessary to so orient them with respect to the field.

Both of the aforementioned copending applications employed means forsquaring all three magnetometer outputs before addingthem together inorder to compensate for the position of the detector system in space.They may, therefore, be called three-element-squared systems andfundamentally the outputs of these systems are proportional to thesquare of the total iield observed. It is sometimes desirable to havethe output vary directly as the first power of the field instead of asthe square without losing the advantages of compensation aiorded by thethreeelement-squared system.

It is an object of this invention to provide a compensated magneticfield detector system which produces an output proportional to the iirstpower of the eld intensity.

The foregoing object is achieved by this invention which comprises incombination a detector magnetometer comprising a core of magneticmaterial having a principal magnetic axis-and two orientingmagnetometers each similar to said detector magnetometer. Windings areplaced on each of said cores and they are supported with their principalaxes mutually perpendicular. A source of alternating current isconnected to windings of each magnetometer whereby even order harmonicvoltages4 are generated therein of magnitudes proportional respectivelyto the product of the field strength and the cosine of the angle formedbetween the principal axis of each core and the direction of the eld. Anorienting means including an electric motor means is connected towindings on said two orienting magnetometers and is responsive toselected even order harmonic voltages genern ated in the orientingmagnetometers for maintaining the principal axis of said detector insubstantial alignment with the direction of the field to be observed. Anelectric squaring means responsive to a selected even order harmonicvoltage generated in each of the two orienting magnetometers produces acompensating direct current varying as a function A`of the sum of theirsquares. The squaring means is coupled to the detector magnetometercircuit so that the compensating current is caused to compensate theresponse of the detector magnetometer to any small angular misalignmentwith the field observed. A linear detector derives a direct current froma selected one of the even order harmonic voltages generated in thedetector magnetometer, the output whereof is substantially proportionalto the iirst power of the strength of the field observed, which outputis read by a direct current indicating instrument or may be recorded ona recorder.

In the copending application of E. P. Felch, F.

G. Merrill and T. Slonczewski, Serial No. 618,550, filed on even dateherewith, there is disclosed and claimed the specific embodiment of thisinvention wherein the compensating current is passed directly throughthe detector magnetometer to correct the responsethereof to any smallangular misalignment" with the magnetic field.

These magnetometers comprise essentially a length of low retentivitymagnetic material preftrably of high permeability and upon which one ormore windings are wound. In the drawings only one winding has beenshown. However, more than one winding obviously may be employed. It hasbeen discovered that when the magnetic eld is at right angles to theprincipal axis of the core and the winding be excited by a voltage offundamental frequency, no even order harmonics will be generated in thewinding. On the oontrary, if the magnetic field has a component in thedirection of the principal axis of the core and the winding be excitedby a voltage of fundamental frequency, even order harmonic voltages willeach be proportional to the cosine of the angle formed by the principalaxis of the magnetometer and the direction of the magnetic eld. Whileany one of these harmonics can be used the second harmonic is selectedin illustrating this invention. i

It can be shown mathematically that the sum of the squares of thesethree second harmonic voltages is entirely independent of theorientation of the three magnetometers with respect to the direction ofthe magnetic eld providing the three magnetometers are retained inmutual perpendicular relationship and they are equally sensitive. Inaccordance with this invention iscan be shown mathematically that if theoutputs of the axial and transverse magnetometers are each squared andthe sum of the squares of the output currents be passed into thedetector magnetometer circuit, it will automatically compensate thechange in output current from the detector magnetometer due to slightmisalignment of the detector magnetometer with respect to the directionof the field. This slight misalignment is shown in Fig. 1 by a smallangle a. It can be shown that this compensation is extremely goodproviding the angle of deviation a is kept relatively small, say withinabout 3 degrees.

The invention may be better understood by referring to the accompanyingdrawings in which:

Fig. 1 discloses an embodiment of the invention in block form;

Figs. 2 and 3 show modifications of the circuit disclosed in Fig. 1;

Fig. 4 shows one kind of squaring rectier which may be employed forsquaring the output of the orienting magnetometer circuit;

Fig. 5 discloses an orienting mechanism` suitable for controlling theposition of the magnetometer elements with respect to the field; and

Fig. 6 shows a preferred form of indicator circuit for the detectormagnetometer channel.

Referring now more particularly to Fig. 1 the three magnetometersmounted in a mutually perpendicular relationship are generally denotedby the reference numeral I. These three magnetometers are called forconvenience the detector magnetometer, the transverse magnetometer andthe axial magnetometer, respectively designated by the referencecharacters Mn, M'r and MA. The transverse and axial magnetometers arealso hereinafter referred to as the orienting magnetometers in that thevoltages derived from these magnetometers are used for orienting the'.de-

tector magnetometer into substantial alignment with the direction of theeld shown in Fig. l by the vector H. As will be hereinafter moreparticularly described the orienting function is such as to" maintainthe detector magnetometer MD ln alignment with the magnetic eld vector Hwithin a small angle a. It will be clearly understood from solidgeometry that if three axes are mutually perpendicular and two of themare kept normal with a particular direction or line. it necessarilyfollows that the third axis will be in alignment with that direction orline. Thus it follows that if the transverse and axial magnetometers Mrand Ma maintain their principal axes normal to the direction of thefield vector H. the direction of the principal axis of the detectormagnetometer Mo will be in alignment with that field. The means wherebya magnetic system of this type may maintain such orientation forms thesubject-matter of the aforesaid application of E. P. Felch and T.Slonczewski. However, for the purposes of this specification it willbe,de scribed briefly later on in this specification. .n

A source oi' alternating current 2 of frequency F is connected to alltnree of the above-mentioned magnetometers through a lter network 3.This network is arranged to pass the fundamental frequency F from thealternating current source 2 and reject all harmonics thereof. 'lhesecurrents of frequency F are passed through the three magnetometers byvway of obvious circuits from the filter 3 including the threeinductances LA, Lr and LD, respectively. It may here be stated that thesubscript letters A, T and D used in connection with the magnetometersand inductances refer to the axial transverse and detector channelsrespectively. For convenience this has been carried throughout Fig. 1not only as subscripts but occasionally in parentheses to denote towhich of the three magnetometers the particular channel isA related.

The currents of frequency F will generate in these magnetometers aseries of harmonics. For purposes of detection in accordance with theprinciples of this invention the even order harmonics are the only onesof interest as these have been found to vary in accordance with thecosine of the angular displacement of the principal axisvof themagnetometer with the direction of the field. These even orderharmonics, for example, the second harmonic of frequency 2F, developsvoltages of that frequency across inductances LA, L'r and Ln. Thecurrents of second harmonic frequency are passed through filters 4 andamplifled by amplifiers 5.

Orientation is achieved in accordance with the principles described inthe aforementioned application of E. P. Felch and T. Slonczewski whichmay be briefly described as follows. The second harmonic output voltagesfrom the axial and transverse channels coming from the amplifiers 5 inthese two channels are carried over circuits B to amplifiers 1. Theoutputs of the two ampliflers 'l are applied to the transverse and axialmodulators 8 where the frequencies are changed to frequency f, amplifiedby amplifiers l0 and applied to windings in the two two-phase motors ofthe orienting mechanism 3l'. The actual construction of this mechanismwill be described in greater detail in connection with Fig. 5. Thesuitable frequency f for driving the motors is derived from themodulators 8 by reason of the modulation of the second harmonicfrequencies 2F and the output of the separateoscillator 9. The actualmodus operandi of this modulation process need not be described as it iswell known in the electronic art.

The outputs from the transverse and axial modulators 3 are applied toone of the phase windings in each of the two orienting motors of theo'rienting mechanism 31. These motors are designated the transversemotor and the axial motor in accordance with the magnetometer to whichthey are responsive. The other phase winding of each of these two motorsis constantly excited from a source of the same frequency f by means ofthe following circuit. A source of frequency F is obtained from thefilter 3 and applied to the frequency doubler II where it isv doubled toa frequency 2F. Actually the frequency doubler producesa number ofharmonics and in order to peak the output of the frequency doubler tothe second harmonic the output from the doubler II is passed through anetwork I2 which readily transmits 'the second harmonic. This may bemost any type of tuned network which may have a parallel antiresonantcircuit, a series resonant circuit, or both. The second harmonic outputof this network I2 is thus made of the same frequency 2F as the outputfrom the two crienting amplifiers 1. Consequently when modulated withthe same source 9 as is used in connection with the output from the twoampliiiers 1 the same frequency ,f is derived.

So long as the axial and transverse magnetometers MA and MT are exactlynormal to the direction of the eld there will be no even order harmoniesgenerated in their windings and consequently there-will be no outputvoltage coming from the axial and transverse amplifiers 5 to betransmitted through the orienting circuit conductors 6 to the modulators8. Therefore, the only voltage applied to the two orienting motors isthat applied to one of the phase windings in each of the motors. Themotors therefore remain stationary. Should, however, either of the twoorienting magnetometers, or both of them, become inclined at an angleslightly different from 90 degrees with the direction of the field byreason of the detector magnetometer becoming slightly misaligned fromthe field, second harmonic voltages proportional thereto will be derivedfrom the axial and transverse amplifiers 5 and transmitted over theorienting circuits 6. This results in voltages being applied to theother phase windings of one or both of the orienting motors producingrotation in the proper direction to return its associated magnetometerinto a position normal with the direction of the field, whereupon themotor will again stop. 'Ihe mechanical connection between the transverseand axial motors is shown schematically by means of dotted lines in Fig.1 indicating that each motor is associated with its own magnetometer tocorrect its position should it become misaligned from the normal withthe direction of the field.

Since the output` of the detector magnetometer MD is proportional to theproduct of the intensity of the field vector H, and the cosine of theangle a it is obvious that some compensation is needed for the output ofthe detector magnetometer when it deviates slightly from exact alignmentwith the eld. This compensation is provided in accordance with theprinciples of this invention by squaring the output from the axial andtransverse channels and applying the sum of the squares of these twooutputs to the detector magnetometer channel.

One way of accomplishing this is shown in Fig. 1 wherein squaringrectifiers I3 are connected to the outputs of the axial and transverseanipliiiers l. These squaring rectifier-s are of a type which produce anoutput direct current proportional to the square of the inputalternating volttage. The output currents from the axial and transversesquaring rectifiers are passed through potentiometers I4 and Il,respectively. A portion oi' the resulting voltages is selected byadjusting the sliders Cs and C'r of these potentiometers to derivevoltages VA and VT, respectively. These voltages are positive at thesliders with respect to ground and produce currents IA and Ir,respectively, through series resistors RA and R'r. Since the upper endsof these two resistors are joined together by conductor I6, these twocurrents are added and comprise a compensating current Ic which ispassed through the detector magnetometer circuit.

In Fig. 1 this compensating current is passed directly through thedetector magnetometer winding in such a direction as to increase theamount of applied field to the magnetometer. Now it will be rememberedthat so long as the detector magnetometer is in exact alignment with thefield there will be no output voltage from Athe axial and transversechannels and consequently the compensating current will be zero. This isas it should be since no compensating current is required. On the otherhand, if a slight misalignment of the detector magnetometer takes placethe compensating current thus developed will increase the fieldsuiiiciently to just compensate for the amount of field component losson the detector magnetometer due to the slight misalignment. Aspreviously stated this compensation can be shown mathematically to beextremely accurate for relatively smallangles and in practice it hasbeen found that the degree of precision in orientation is suflicientlygood so'that the com pensation allows no appreciable error in.detection.

In order to measure the response of the de .tector magnetcmeter Mn tothe strength of the field the output thereof is passed through its'second harmonic lter 4 in the detector channel (D), through conductor34 and is amplied in amplifier 21. The output of this amplier is applied-to a linear detector 28 so that the direct current output therefrom isdirectly proportional to the first power of the second harmonic voltagegenerated in the detector magnetometer. This output current In is passedthrough a speed control network 29, which will be more particularlydescribed later in connection with Fig. 6, and applied to the inputcircuit of a direct current amplifier 30. the output of which isconnected to a suitable recorder 3|, visu-al indicator 32 or both. Asensitivity control 33 may be connected in series in order to adjust theresponse of the indicators. Since the output of the detectormagnetometer has been linearly detected by detector 28, the indicationwill be in proportion to the first power of the intensity of the totaleld.

considerably increased precision to changes in the magnitude of thetotal fleld may be achieved by balancing out part of the earths eldapplied to the detector magnetometer Mn. This.y is achieved by means ofthe eld biasing network I 9 which comprises a stable direct currentsource 20 and potentiometers 2l and 22. Potentiometer 22 provides acoarse adjustment of voltage at its current source 2U is in reversesense with respect to ground as compared with the polarity of theoutputs from the squaring rectiers I3. Field bias source 20 thereforeprovides a eld bias current In through the detector magnetometer Mn in adirection to reduce considerably the amount of field applied to thedetector magnetometer. This field bias may be adjusted to anypredetermined desired amount. In practice, it has been found quitedesirable to balance out about 90 per cent of the eld applied by theearths magnetism. This increases considerably the accuracy with whichthe equipment may detect small changes in the earths field or any othereld in which the magnetometer system may be immersed. For example, witha iield intensity in the order of 55,000 gamma the eld bias adjustingmeans I9 is adjusted to balance out all but about 5,000 gamma so thatthe detector magnetometer is required to detect changes in only 5,000gamma rather than changes in 55,000 gamma, thereby in` creasingconsiderably the accuracy with which the changes may be detected. It is,of course, obvious that the field bias means I9 may be adjusted tobalance out different amounts of the total eld.

In the particular embodiment specically shown in Fig. 1 the compensatingcurrent Ic is collected in conductor I6 from the axial and transversecurrents IA and I'r. respectively. This compensating current is thenadded algebraically to the field bias current IB and by way of conductor38 and terminal I1 it is passed through the detector magnetometerwinding to combine the eiect of the compensating current and the eldbias current in the detector magnetometer circuit.

A rheostat 25 is shown in series with conductor I6 and conductor 38.This rheostat adjusts the veiect of the compensating current on thedetector magnetometer and adjusts the system to operate correctly withfields of different intensity. In using a two-element square system ofthe type disclosed in this invention it is necessary that thecompensating current be adjusted for different field intensities to bemeasured. Consequently, when the 'apparatus is being adjusted for zeroindication in a particular field the coarse adjustin g potentiometer 22of field bias means I9 is adjusted to balance out a suilcient amount ofthe eld to bring the indicator to zero. If the apparatus is beingadjusted to zero in a different fleldintensity, potentiometer 22 must beadjusted to a diierent point. Therefore, both rheostat 25 andpotentiometer 22 must be adjusted in accordance with the intensity ofthe field to be measured and are consequently ganged together through acommon control 26 so that as the eld bias control is adjusted to changethe position of potentiometer 22 the series rheostat 25 is automaticallyand simultaneously adjusted to correct the effect of the compensatingcurrent for the diierent eld strengths being measured.

If it is desired to measure the total absolute eld rather than theresidual unbalanced portion thereof, it is unnecessary to use the eldbias control means I9. It is also unnecessary to use a series rheostat25 as shown in Fig. 2 as the effect of the compensating current may beinde ure the total absolute eld. In this figure it will be noted thatthe eld bias control means I9 has been eliminated as has also therheostat 25 and that the connection 38 may be connected to eitherterminal I'I or terminal I8 of Fig. 1. Terminal I8 in Fig. 1 is locatedin the detector channel between the linear detector 28 and the speedcontrol circuit 29. It will thus be seen that the connection for thecompensating current may be ymade to either of two pla-ces in thedetector channel. The connection to terminal II is somewhat preferredsince it reduces the stability requirements on the variousy ampliers andtransfers them to the field bias battery source 20. Otherwise the eiIectof using either terminal II or terminal I8 is substantially the same.When the connection is made to terminal I8 the compensating currentwhich is, of course, proportional to the sum of the squares of the twoorienting magnetometer outputs is added to the lineraly detected outputof the detector magnetometer. It can be shown mathematically that theeffect of this connection is substantially the same as the effect of theconnection to terminal I1.

If it is desired that the compensating current be added to the linearlydetected current from detector 28, that is, the compensating currentconductor I6 is connected to terminal I3 and it is also desired tobalance out a large part of the total absolute field, the circuits ofFig. 3 may be used to modify Fig. 1. In this case the eld bias controlcircuit I9 is connected to the detector magnetomcter by way of conductor38 and terminal I1, while the compensating current derived from the sumof the squares of the outputs of the two squaring rectiiiers I3 isconveyed by way of conductor I6 to terminal I8.

Squaring rectiers I3 may be of any type well known in the art as, forexample, they may be of the harmonic generator type disclosed in thecopending application of E. P. Felch and T. Slonczewski filed April 20,1943, Serial No. 483,754, now Patent No. 2,427,666, or they may employparabolic or square law rectiiiers of the type disclosed in theaforementioned copending application of T. Slonczewski, the latter beingof the same general type shown in Fig. 4 of this application.

In Fig. 4 the squaring rectifier circuit for one of the squaringrectifiers I3 of Fig. 1 is disclosed.

In this gure the second harmonic output of thev axial magnetometerchannel is transmitted to the input circuits of the rectifier by way ofconductor 36. A tuned circuit 39, broadly tuned to the second harmonic,is connected across the input circuit to the rectifier. This rectierutilizes a twin triode 40. In the description of the operation of thiscircuit We may neglect for the moment the effect of the triode section40A. The rectifier proper is actually associated with the triode section40B. This is a conventional square law rectier operating on theprinciple of plate rectification. The voltage of second harmonicfrequency is applied to the grid circuit through condenser -III and bymeans of plate rectication the plate current flowing through resistor44, resistor 43 and potentiometer I 4 will vary in magnitude as thesquare of the applied alternating voltage to the grid circuit.Consequently the output voltage VA appearing between the slider controlCA of potentiometer I4 and ground is proportional to the square of theinput second harmonic voltage.

It has been found that the plate current of such a rectier is not only afunction of the square of the magnitude of the input voltage but is alsoa function of the temperature of the cathode. As the temperature of thecathode increases the plate current also increases as 9 is well known inthe art. .To compensate for this variation the triode section 40A isconnected as a diode in series with resistance 45. The emission from thecathode in the diode section 40A develops a space current across thespace between the anode and cathode, which space current develops avoltage across condenser 41 and resistor A6 with the polarity asindicated in Fig. 4. This voltage varies with the temperature of thecathode, increasing with increasing temperature. It will be noted thatresistor 4B is connected in series with condenser 42 and resistor 45thereby adding to the bias on the grid of the triode section 60B. Bothcathodes are heated in a conventional manner from the same source ofcurrent, not shown, so that as the heater current increases or decreasesslightly, the temperatures of the two cathodes will tend to remainequal. As has already been stated. an increase in cathode temperaturewill cause an increase in the plate current through the triode section40B but this increase in plate current can be compensated by theincrease in bias voltage generated in the diode section A150A andconversely. the decrease in cathode temperature will be similarlycompensated. This increases considerably the accuracy with which thesquaring rectifier will produce an output direct current proportional tothe square of the input applied alternating voltage. The inventiondisclosed in this compensating means form the basis for the copendingapplication of C. H. Young, Serial No. 618,549, flied on even dateherewith.

The three magnetometers may be mounted on most any kind of structurewhich will hold them in mutually perpendicular relationship. Thisstructure should then be mounted as a rotor in a gimbal mechanism sothat the magnetometer elements may be freely adjusted to any position inspace. A preferred form of such mechanism is disclosed in the copendingapplication of W. J'. Means, iiled July 30. 1943. Serial No. 496.833,now Patent No. 2,427,014. For convenience this mechanism is alsodisclosed in Fie. 5 wherein the gimbal structure i is shown at the leftand the two drive motors for the transverse and axial channels are shownat the right. The two drive motors are coupled to the gimbal structurethrough mechanical drives in the form of flexible cables. In the gimbalmechanism i the inner rotor supporting structure 48 is shown mounted onbearings B9 and 50. The shafts for these bearings are secured to aninner ring 5i which in turn is supported on two hollow shafts 52 and 53secured to the outer ring 5d. It will thus be seen that by means of thebearings 49 and 50, the axis of rotation of which is at right angleswith respect to that of shafts 52 and 53. the inner rotor d8 may bepositioned so that a diameter thereof may take any direction in space.Since the detector magnetometer MD lies n a diameter of the rotor 48 asshown in Fig. 5 the detector magnetometer may thereby be rotated inalignment with the eld regardless of the direction of the eld in space.The two orienting magnetometers, designated the axial magnetometer MAand the transverse magnetometer MT, are so designated by reason of theirpositions with respect to the principal axis of the rotor 48.

It will be seen in Fig. that the axial magnetometer MA lies along theIaxis of rotation of the rotor 43 whereas the transverse magnetometer istransverse to that axis. The transverse motor drives the rotor 48 aboutits axis on bearings 9 and B0 through a mechanical drive comprising aflexible cable passing over the motor pulley through the hollow shafts52 and 53 and around a pulley 55 attached to the rotor 48. It will thusbe seen that whenever the transverse magnetometer Mr lies at any angleless than degrees with respect to the -direction of the field, thetransverse motor may drive the rotor B8 until the transversemagnetometer is again realigned with the normal to the field. A similardriving larrangement is provided yfor the axial magnetometer wherein theaxial motor drives the inner ring 5| about its axis defined by thebearings 52 and 53 through a ilexible cable passing over the axial motorpulley and a pulley 56 attached to the inner ring 5i. The action of theaxial magnetorneter in causing the axial motor to rotate this inner ring5i is similar to that already described for the transversemagnetorneter. It will be understood that by reason of the combinedaction of the two orienting magnetometers, the detector magnetometer maybe oriented into alignment with the direction of the field andautomatically maintained in that position regardless of any shifting ofthe fiel-d itself or shifting of the mechanical structure with respectto the field.

The electrical connections to the three magnetometers are made throughslip rings and brushes 51. One of these brushes connects to groundwhereas the other three connect to the three magnetometers as shown inFig. 1. The brushes are mounted on the inner ring 5i and connectionsfrom them are carried to another group of four slip rings divided forconvenience into two parts 5t and 59. The brushes of these four sliprings are attached to the stationary outer ring 54 and the electricalconnections are thereby easily connected to the external circuit bvmeans not shown in Fig. 5 but the circuits therefor are clearly shown inFig. 1.

More detailed circuits for tbe detector channel are shown in Fig. 6.Comparing this figure with Fig. 1 it will be noted that the output ofthe second harmonic filter coming from the' detector magnetometer istransmitted to the input circuit of amplifier 21 by way oi conductor 3LThese reference numerals are shown on both cures. In Fig. 6 it will benoted that the amnlier 21 comprises two stages with a gain control 35.The amplifier is generally of conventional form and need not bedescribed in detail except that a control rheostat 6U has been providedto adjust the screen grid voltage until the over-all gain of theamplifier is compensated automatically for variations in plate supplyvoltage. When this adjustment is properly marie small variations` inplate supplv voltage will not appreciably aiect the gain of thisamplier.

The output of the amplifier 21 is apcled to a linear detector circuit 2Rof ccnventional form. The direct current output of this detector appearsas a voltage across condenser Gland resistor 62, with the polarity Iasindicated in Fig. 6. This output direct voltage is opposed by a fixeddirect voltage across resistor 64 derived from current from source 66through resistor 65. This latter The difference voltage appearing acrossresistors 62 and 64 in series will vary in proportion to the magnitudeof the second harmonic voltage coming from detector magnetometer Mn.This voltage is applied to the input circuit of a direct currentamplifier 30 through a speed control network 29. This speed controlnetwork comprises two resistors 61 and 68 which are relatively highcompared to a shunt resistor 69 and .three parallel networks 10, 1I and12, respectively, which may be switched at will -across the relativelyhigh resistors 61 and 68. Networks 10 1I and 12 are switched by means ofa suitable switch 13 which, if switched to its upper terminal willconnect resistor 10 across high resistors 61 and 68. In this positionthe system is sensitive to the magnitude of a static field. Sometimes,however, it is desirable -to have the Iapparatus responsive only torapid changes in a field. This is particularly true where the system isbeing carried by aircraft at considerable speed. In such cases theswitch 13 is switched to either of' the other two circuits dependingupon the speed that the aircraft is expected to travel. When switched toeither of these latter two positions it will be noted that only analternating current path exists, except for networks 31, 68 and 69,between the switch 13 and the right-hand end of resistor 68 therebyrendering the circuit responsive only to rapid changes in the strengthof the field. If the switch 13 is on either of the two positions wherenetwork 1I or 12 is in circuit and the aircraft is held stationary as,for example, an aircraft of lighter than air type, or the aircraft ismoving in a oonstant field, the input voltage to the direct currentamplifier 30 will be reduced to a relatively low value determined by thenetwork of resistances 61, 68 and 69. This latter resistance networkprovides a low sensitivity direct current path to the upper grid of twintriode 14 since resistors 61 and 68 are high compared with resistor 63.This gives an indication on indicator 32 of large slow changes in thefield t give warning of ap preaching overload on amplifier 21. y

The two-stage direct current amplifier 30 comprising twin triodes 14 and15 requires very little description. When the input Voltage is appliedto the upper grid of tube 14 a voltage appears across resistor 16 in thecommon cathode circuit. This voltage is applied to the lower grid of thetwin triode 14 in opposite sense and therefore this section of tube 14acts as an inverter. The input circuitcof tube 15 is direct coupled tothe output circuit of tube 14 and the difference voltage appearingacross the cathode resistors 11 and 18 is applied to the indicatorcircuit including the indicator 32 and sensitivity adjusting rheostat33. A recorder may also be included in this circuit if desired as shownin Fig. 1.

Having described the circuits of this invention in considerable detailthe method of setting them up for operation will be described. While thedescription is conned to the particular embodiment specifically shown inFig. l 'it w ll be obvious to those skilled in the art that the samekind of adjustments will set the apparatus up for the modified circuitsshown in Figs. 2 and 3 consideration of course being given to the factthat in Fig. 2 no eld bias network I9 is employed.

It may be assumed that the power is all turned on and that allamplifiers are at their normal operating temperature. The sensitivitycontrol 33 should be adjusted for minimum sensitivity, that is, withmaximum resistance in the circuit. speed control 29 should be adjustedfor static The 2 field operation, that is, with switch 13 shown in Fig.6 on its upper contact. 'I'he field bias coi1s trol knob 26 should nowbe adjusted until indicator 32 reads approximately zero. The sensitivitycontrol 33 should be readjusted to increase the sensitivity after whichthe fine control 2| will trim the f'leld bias to the point wheresubstantially zero output is obtained from amplifier 30 and meter 32reads zero.

The control knob 26 gangs rheostat 25 and the coarse field biasadjusting potentiometer 22 so that when the field bias controlpotentiometer 22 increases the amount of field bias current IB, theresistance Re of rheostat 25 is also simultaneously increased. By thusadjusting the field bias current it will be obvious that a net amount offield applied to the detect-or magnetometer MD will be reached where itsoutput as detected by the linear detector 28 will produce a voltagewhich will just balance the offset Voltage appearing across resistor 64shown in Fig. 6. This condition will be reached preferably when a largepredetermined percentage, say about per cent, of the total field isbalanced out by the bias current in the detector magnetometer. It willbe obvious that when this output voltage appearing across resistor 62 inFig. 6 just balances the offset voltage across resistor 64, no inputdirect voltage will be appled to the direct currentamplier 30 andconsequently the output Voltage, assuming the circuit to be properlybalanced in the amplifier, will be zero and the meter will indicatezero.

To take maximum advantage of the field bias feature a large percentageof the field must be balanced out. The percentage which must -bebalanced out to get a zero indication is determined by the prev;ouslydescribed offset voltage across resistor 64. The lower this voltage thegreater'is the percentage of field baanced out. However, as indicator 32is to be of the zero center type to give indications of direction offield variation as well as the magnitude thereof, this offset Voltageshould not be so small that the variations of field encountered inmaking measurements will reduce the initial net field in the detectormagnetometer MD sufficiently to reverse it. This is because a reversalof field in the detector will not produce a reversed potential acrossresistor 62 due to the fact that the output polarity of detector 28 doesnot reverse as the phase of the detector magnetometer output voltagereverses. Consequently, in one direction of meter deflection anambiguity in readings will take place unless the offset voltage acrossresistor 64 is greater than the changein voltage which will appearacross resistor 62 due to the largest fieid change which will be met.This feature is included in the aforementioned application of E P.Felch, F. G. Merrill and T. Slonczewski, filed on even date herewith. I

It is assumed that the direction of the field applied to the detectormagnetometer MD does not vary and, therefore, the deviation angle ashown in Fig. 1 should remain substantially zero. Consequently, nocompensating current output should be derived from the squaring rectiersI3. With the magnetometer system thus properly oriented the twoorienting motors should be rendered incapable of moving the orientingmechanism. Of course, the orienting mechanism should be temporarilyfixed stationary so as not to misalign the detector magnetometer whilemaking the adjustments. The purpose for doing this is to aid inadjusting the compensating current output from the two squaririgrectiers I3.

The detector magnetometer should then be moved through a small knownangle about the principal axis of the transverse magnetometer (or aboutan axis parallel thereto) thereby displacing the axial magnetometer anequal amount. The amount of this defiectionshould be in the order o! 1to 2 degrees and should be carefully determined by any suitable means,not shown. Assuming that the compensating current potentiometers I4 andl5 are out of adjustment this small dis-placement of the detectormagnetometer will cause the indicating instruments 3| and 32 toYdeilect. The potentiometer Il should have its slider CA adiusted untilthe detector magnetometer may be rotated equal small angles about theprincipal axis of the transverse magnetometer without causingr adeection from zero position on the indicating instrument. A similaradjustment is made for the transverse magnetometer by deecting thedetector magnetometer back and forth about the principal axis of theaxial magnetometer and adjusting the slider Cr of potentiometer i5 untilno deilection is observed in the indicating instruments. With theapparatus thus adjusted the orienting motors should again be permittedto drive 'the magnetometers in response to the output of the twoorienting magnetometers.

The gain control 35 should be adjusted so that amplifier Z1 is operatingabout midway the linear part of its characteristic when the input toamplier Sil is zero.

It is only necessary now to change the amount caused tn compensate theresponse of the detecof field applied to the detector magnetometer Mn byknown amounts and adjust the sensitivity control 33 until the indicatinginstruments 3l and 32 read correctly the amount of change in themagnetic eld applied to the detector magnetometer.

No adjustment of the resistances in the eld bias control means i9 orrheostat 25 is necessary since by merely predetermining themagnetometive force generated in the detector magnetom eter per ampereof direct current therethrough the various resistances in these networksmay be very accurately calculated.

What is claimed is:

l. A system for indicating changes in the strength of a magnetic fieldcomprising in combination a detector magnetometer comprising a core ofmagnetic material having a principal magnetic axis, two orientingmagnetometers each similar to said detector magnetometer, windings ineach of said cores, means supporting said three magnetometers with theirprincipal axes mutually perpendicular, a source of alternating currentconnected to windings of each magnetometer whereby even order harmonicvoltages are generated therein of magnitudes proportional respectivelyto the product of the eld strength and the cosine of the angle formedbetween the principal axis of each core and the direction of the eld, anorienting means including electric motor means connected to windings onsaid two orienting magnetometers and responsive to selected even orderharmonic voltages generated therein for maintaining the principal axisof said detector in substantial alignment with the direction of thefield to be observed, an electric squaring means responsive to aselected even order harmonic voltage generated in each of the twoorienting magnetometers for producing a compensating direct currentvarying as a function of the sum of their squares, circuits coupling thesquaring means to the detector magnetometer circuit whereby thecompensating direct current is' tor magnetometer to any small angularmisalignment with the eld observed, a linear detector for deriving adirect current from a selected one of said even order harmonic voltagesgenerated in said detector magnetometer and an indicating meansresponsive to the compensated direct current output of the lineardetector.

2. A system for indicating changes in the strength of a magnetic fieldcomprising in combination a detector magnetometer comprising a core ofmagnetic material having a principal magnetic axis, two orientingmagnetometers each similar to said detector magnetometer, windings oneach or said cores, means supporting said three magnetometers with theirprincipal axes mutually perpendicular, a source of alternating currentconnected to the windings of each magnetometer whereby even orderharmonic voltages are generated therein of magnitudes proportional.respectively, to the product of the field strength and the cosine of theangle formed between the principal axis of each core and the directionof the ileld, an orienting means including electric motor meansconnected to windings on said two orienting magnetometers and responsiveto selected even order harmonic voltages generated therein formaintaining the principal axis of said detector in substantial alignmentwith the direction of the field to be observed, an electric squaringmeans responsive to a selected even order harmonic voltage generated ineach of the two orienting magnetometers for producing a com= pensatingdirect current varying as a function of the sum of their squares,circuits coupling the squaring means to the detector magnetometercircuit whereby thecompensating direct current is caused to ilow intothe detector circuit to compensate it for any small angular misalignmentwith the eld observed, a linear detector for de.- riving adirect'current from a selected one of said evenorder harmonic voltagesgenerated in said detector magnetometer and an indicating meansresponsive to the compensated direct current output of the lineardetector.

3. A system for indicating changes in the strength of the magnetic eldcomprising in combination a detector magnetometer comprising a core ofmagnetic material having a principal magnetic axis, two orientingmagnetometers each similar to said detector magnetometer, windings oneach oi said cores, means supporting said three magnetometers with theirprincipal axes mutually perpendicular, a source of alternating currentconnected to windings` of each magnetometer whereby even order harmonicvoltages are generated therein of magnitudes proportional respectivelyto the product of the field strength and the cosine of the angle formedbetween the principal axis of each core and the direction of the eld,orienting means including electric motor means connected to windings onsaid two orienting magnetometers and responsive to selected even orderharmonic voltages generated therein for maintaining the principal axisof said detector in substantial alignment with the direction of thefield to be observed, a separate electric squaring means connected toeach of the two orienting magnetometer circuits to produce two directcurrents each proportional to the square of a selected even orderharmonic voltage generated in each of said orienting magnetometersrespectively, a combining circuit for combining said two squaredcurrents into a single compensating current, circuits connecting thecombining circuit to the detector magnetometer circuit whereby thecompensating direct current is caused tocompensate the response of thedetector magnetometer to any small angular misalign-` ment with the eldobserved, alinear detector for deriving a direct current from a selectedone of said even order harmonic voltages generated in said detectormagnetometer and an indicating means responsive to the compensatingdirect current output of the linear detector.

4. A system for indicating changes in the strength of a magnetic fieldcomprising in combination a detector magnetometer comprising a core ofmagnetic material having a principal mag- `netic axis, two orientingmagnetometersA each similar to said detector magnetometer, windings oneach of said cores, means supporting said three `magnetometers withtheir principal axes mutually perpendicular, a source of alternatingcurrent connected to the windings of each magnetometer whereby evenorder harmonic voltages are generated therein of magnitudes proportionalrespectively to the product of the eld strength andthe cosine of theangle formed between thc principal axis of each core and the directionof the iield, an orienting meanslincluding electric motor meansconnected to windings on said two orienting magnetometers andresponsiveto selected even order harmonic voltages generated therein formaintaining the principal axis of said detector in substantial alignmentwith the direction of the field to be observed, a separate electricsquaring means connected to each of the` two orienting magnetometercircuits to produce two `direct currents each proportional to the squareof f `a selected one of said even order harmonic voit- `ages generatedin said detector magnetometer, land an indicating means responsive tothe compensated direct current output of the linear detector.

5. A system for indicating changes in the strength of a magnetic iieldcomprising in combination a detector magnetometer comprising a core ofmagnetic material having a principal magnetic axis, two orientingmagnetometers each n similar to said detector magnetometer, windings oneach of said cores, means supporting said three 1 magnetometers withtheir principal axes mutually perpendicular, a source of alternatingcurrent connected to the windings of each magnetometer whereby evenorder harmonic voltages are generated therein of magnitudes proportionalrespectively to the product of the ileld strength and the cosine of theangle formed between the principal axis of each core and the directionof the field, an orienting means including electric motor meansconnected to windings on said two orienting magnetometers and responsiveto selected even order `harmonic voltages generated therein formaintaining the principal axis of said detector in substantial alignmentwith the direction of the field to be observed, an electric squaringmeans responsive to a selected even order harmonic voltage generated ineach of the two orienting magnetometers for producing a compensatingdirect current varying as a function of the sum of their squares, alinear detector for deriving a direct current from a selected one ofsaid even order harmonic voltages generated in said detectormagnetometer, a combining circuit for combining the compensating currentand the linearly detected current into one output current whereby theresponse of the detector magnetometer is compensated for any smallangular misalignment with the field observed, and an indicating meanscoupled to said combining circuit and responsive to said output current.

6. A system for indicating changesY in the strength of a magnetic fieldcomprising in combination a detector magnetometer comprising a core ofmagnetic material having a principal magnetic axis, two orientingmagnetometers each similar to said detector magnetometer, windings oneach of said cores, means supporting said three magnetometers with theirprincipal axes mutually perpendicular, a source of alternating currentconnected to windings of each magnetometer whereby even order harmonicvoltages are gener ated therein of magnitudes proportional respectivelyto the product of the eld strength and the cosine of the angle formedbetween the principal axis of each core and the direction of the eld, anorienting means including electric motor means connected to windings onsaid two orienting magnetometers and responsive to selected even orderharmonic voltages generated therein `for maintaining the principal axisof said detecf duce a single output current compensated for any smallangular misalignment ofthe detector magnetometer with the eld observed,and an indicating means coupled to said combining circuit and responsiveto said output current.

7. A system for indicating changes in the strength of a magnetic iieldcomprising in combination a detector magnetometer comprising a core ofmagnetic material having a principal magnetic axis, two orientingmagnetometers each similar to said detector magnetometer, windings oneach of said cores, means supporting said three magnetometers with theirprincipal axes mutually perpendicular, a source of alternating currentconnected to windings of each magnetometer whereby even order harmonicvoltages are generated therein of magnitudes proportional respectivelyto the product of the eld strength and the cosine of the angle formedbetween the principal axis of each core and the direction of the field,an orienting means including electric motor means connected to windingson said two orienting magnetometers and responsive to selected evenorder harmonic voltages generated therein for maintaining the principalaxis of said detector in substantial alignment with the direction of thefield to be observed, a separate electric squaring means connected toeach of the two orienting magnetometer circuits to produce two directcurrents each proportional to the square of a selected even orderharmonic voltage generated in each of said orienting magnetometers,respectively. a linear detector for deriving a direct current from aselected one of said even order harmonic volt ages generated in saiddetector magnetometer, a rst combining circuit for combining said twosquared currents into a single compensating current, a second combiningcircuit for combining the compensating current and the linearly detectedcurrent to produce a. single output current compensated for any smallangular ent lof the detector magnetometer with the eld observed, and anindicating means coupled to said second combining circuit and responsiveto said output current.

THADDEUS SLONCZEWBKI.

18 REFERENCES crrnn The following references are of record ln the me ofthis patent:

UNITED STATES PATENTS v Cunningham et al. Apr. 6, 1948 OTHER REFERENCESTransactions American Institute of Mining and Metallurgical Engineers,Geophysical Prospecting,

u 1932. pages 213 and 214.

